My only advice would be to ignore it.
I get them on my iPhone all the time: notifications of messages that I responded to hours ago, 'ship now' on things already marked mailed. Check and ignore. The notifications are a bit buggy, to put it mildly.
The mobile device apps were made with only buyers in mind and offer merely frustration for sellers. I never EVER use mine to do anything important related to selling. Only monitor. I had to issue a partial refund of about $6 on a $60-order to a buyer for a combined shipping invoice that was never issued to them (because they used the app to buy four things from me) and ever since that night which was more than a month ago, the app is telling me the buyer didn't pay.
On my desktop, everything appears as it should be and there is nothing wrong with the Sales Records for those items. Paid and Shipped.
Only the app thinks the whole order is unpaid which, of course, it is not. It was delivered a month ago. I can do nothing but ignore it.